arXiv subjects

Pedro Tarazona

Publications and source records attributed to Pedro Tarazona.

8 recordsLinked to original sources

arXiv2025

Dynamics and rupture of doped Motility Induced Phase Peparation

Adding a small amount of passive (Brownian) particles to a two-dimensional dense suspension of repulsive active Brownian particles does not affect the appearance of a motility-induced phase separation into a dense and a dilute phase, caused by the persistence of the active particles' direction of motion. Unlike a purely active suspension, the dense slab formed in an elongated system of a passive-active mixture may show, over long periods of time, a stable and well-defined propagation of the interfaces, because of the symmetry breaking caused by the depletion of passive particles on one side of the slab. We investigate these dynamical structures via average density profile calculations, revealing an asymmetry between the two interfaces, and enabling a kinetic analysis of the slab movement. The apparent movement of the dense slab is not a pure source/sink effect, nor a rigid displacement of all the particles, but a self-sustained combination of both effects. Furthermore, we analyse the specific fluctuations that produce, cancel and abruptly reverse the slab motion.

cond-mat.soft↗

arXiv2022

Intrinsic structure perspective for MIPS interfaces in two dimensional systems of Active Brownian Particles

Suspensions of Active Brownian Particles (ABP) undergo motility induced phase separation (MIPS) over a wide range of mean density and activity strength [1], even in the absence of an explicit attraction. Negative values of the mechanical surface tension have been reported, from the total forces across the interface, while the stable fluctuations of the interfacial line would be interpreted as a positive capillary surface tension [2], while in equilibrium liquid surfaces these two magnitudes are equal. We present here the analysis of 2D-ABP interfaces in terms of the intrinsic density and force profiles, calculated with the particle distance to the instantaneous interfacial line. Our results provide a new insight in the origin of the MIPS from the local rectification of the random active force on the particles near the interface. As it had been pointed, that effect acts as an external potential [3] that produces a pressure gradient across the interface, so that the mechanical surface tension of the MIPS cannot be described as that of equilibrium coexisting phases; but our analysis shows that most of that effect comes from the tightly caged particles at the dense (inner) side of the MIPS interface, rather than from the free moving particles at the outer side that collide with the dense cluster. Moreover, a clear correlation appears between the decay of the hexatic order parameter at the dense slab and the end of the MIPS as the strength of the active force is lowered. We test that with the strong active forces required for MIPS, the interfacial structure and properties are very similar for ABP with purely repulsive (WCA-LJ model truncated at its minimum) and when the interaction includes a range of the LJ attractive force.

cond-mat.soft↗

arXiv2017

Deconstructing temperature gradients across fluid interfaces: the structural origin of the thermal resistance of liquid-vapor interfaces

The interfacial thermal resistance determines condensation-evaporation processes and thermal transport across material-fluid interfaces. Despite its importance in transport processes, the interfacial structure responsible for the thermal resistance is still unknown. By combining non-equilibrium molecular dynamics simulations and interfacial analyses that remove the interfacial thermal fluctuations we show that the thermal resistance of liquid-vapor interfaces is connected to a low density fluid layer that is adsorbed at the liquid surface. This thermal resistance layer (TRL) defines the boundary where the thermal transport mechanism changes from that of gases (ballistic) to that characteristic of dense liquids, dominated by frequent particle collisions involving very short mean free paths. We show that the thermal conductance is proportional to the number of atoms adsorbed in the TRL, and hence we explain the structural origin of the thermal resistance in liquid-vapor interfaces.

cond-mat.soft↗

arXiv2008

Beyond the Dynamic Density Functional theory for steady currents. Application to driven colloidal particles in a channel

Motivated by recent studies on the dynamics of colloidal solutions in narrow channels, we consider the steady state properties of an assembly of non interacting particles subject to the action of a traveling potential moving at a constant speed while the solvent is modeled by a heat bath at rest in the laboratory frame. Since the description, we propose here, takes into account the inertia of the colloidal particles it is necessary to consider the evolution of both positions and momenta and study the governing equation for the one-particle phase-space distribution. We first derive the asymptotic form of its solutions as an expansion in Hermite polynomials and their generic properties, such as the force and energy balance and then we particularize our study to the case of an inverted parabolic potential barrier. We obtain numerically the steady state density and temperature profile and show that the expansion is rapidly convergent for large values of the friction constant and small drifting velocities. The present results on the one hand confirm the previous studies based on the dynamic density functional theory (DDFT) when the friction constant is large, on the other hand display effects such as the presence of a wake behind the barrier and a strong inhomogeneity in the temperature field which are beyond the DDFT description.

cond-mat.soft↗

arXiv2007

Diffusion at the liquid-vapor interface

Recently, the intrinsic sampling method has been developed in order to obtain, from molecular simulations, the intrinsic structure of the liquid-vapor interface that is presupposed in the classical capillary wave theory. Our purpose here is to study dynamical processes at the liquid-vapor interface, since this method allows tracking down and analyzing the movement of surface molecules, thus providing, with great accuracy, dynamical information on molecules that are "at" the interface. We present results for the coefficients for diffusion parallel and perpendicular to the liquid-vapor interface of the Lennard-Jones fluid, as well as other time and length parameters that characterize the diffusion process in this system. We also obtain statistics of permanence and residence time. The generality of our results is tested by varying the system size and the temperature; for the later case, an existing model for alkali metals is also considered. Our main conclusion is that, even if diffusion coefficients can still be computed, the turnover processes, by which molecules enter and leave the intrinsic surface, are as important as diffusion. For example, the typical time required for a molecule to traverse a molecular diameter is very similar to its residence time at the surface.

cond-mat.soft↗

arXiv2007

Theory of thermostatted inhomogeneous granular fluids: a self-consistent density functional description

The authors present a study of the non equilibrium statistical properties of a one dimensional hard-rod fluid dissipating energy via inelastic collisions and subject to the action of a Gaussian heat bath, simulating an external driving mechanism. They show that the description of the fluid based on the one-particle phase-space reduced distribution function, in principle necessary because of the presence of velocity dependent collisional dissipation, can be contracted to a simpler description in configurational space. Indeed, by means of a multiple-time scale method the authors derive a self-consistent governing equation for the particle density distribution function. This equation is similar to the dynamic density functional equation employed in the study of colloids, but contains additional terms taking into account the inelastic nature of the fluid. Such terms cannot be derived from a Liapunov generating functional and contribute not only to the relaxational properties, but also to the non equilibrium steady state properties. A validation of the theory against molecular dynamics simulations is presented in a series of cases, and good agreement is found.

cond-mat.mtrl-sci↗

arXiv2002

Close to the edge of Fundamental Measure Theory: density functional for hard sphere mixtures

We analyze the structure of the Fundamental Measure Theory for the free energy density functional of hard sphere mixtures. A comparative study of the different versions of the theory, and other density functional approaches, is done in terms of their generic form for the three-points direct correlation function, which shows clearly the main advantages and problems of the different approximations. A recently developed version for the monocomponent case is extended to mixtures of hard spheres with different radii, and a new prescription is presented to obtain the exact dimensional crossover of those mixtures in the one-dimensional (1D) limit. Numerical results for planar wall-fluid interfaces and for the 1D fluid are presented.

cond-mat.soft↗

arXiv1998

Dynamic Density Functional Theory of Fluids

We present a new time-dependent Density Functional approach to study the relaxational dynamics of an assembly of interacting particles subject to thermal noise. Starting from the Langevin stochastic equations of motion for the velocities of the particles we are able by means of an approximated closure to derive a self-consistent deterministic equation for the temporal evolution of the average particle density. The closure is equivalent to assuming that the equal-time two-point correlation function out of equilibrium has the same properties as its equilibrium version. The changes in time of the density depend on the functional derivatives of the grand canonical free energy functional $F[ρ]$ of the system. In particular the static solutions of the equation for the density correspond to the exact equilibrium profiles provided one is able to determine the exact form of $F[ρ]$. In order to assess the validity of our approach we performed a comparison between the Langevin dynamics and the dynamic density functional method for a one-dimensional hard-rod system in three relevant cases and found remarkable agreement, with some interesting exceptions, which are discussed and explained. In addition, we consider the case where one is forced to use an approximate form of $F[ρ]$. Finally we compare the present method with the stochastic equation for the density proposed by other authors [Kawasaki,Kirkpatrick etc.] and discuss the role of the thermal fluctuations.

cond-mat.stat-mech↗